compressing pdfs without losing quality

Very rarely you will find uncompressed PDFs.

bentopdf.com

I use pdfgear or convert to JPG then resize then reconvert to PDF if it's mostly images

ocrmypdf has built-in lossless compression, with optional lossy compression. But PDFs are pretty light to begin with, unless you have high resolution original quality scans of encyclopedias...

PDFs (even thousands of them) don't need shrinking - their size is trivial. You have a different issue.

You could try pdf24 (has an offline version if you download PDF Creator on their page), or even Adobe Acrobat - but if they compress the pdf significantly, they likely also compress and lower the quality of any embedded images in your pdf, which you might want to avoid.

If it's a scan, there's something you can do.

You can decode PDF into images. If they're huge - resize them. Then encode images back into PDF.

ABBYY Finereader decodes PDFs well.

To resize images fast (batch) - IrfanView (free program).

To encode - either Finereader (70% compression) or LuraTech PDF Compressor (it's on Internet Archive).

I have found that using Adobe is actually the best for compression:

https://www.adobe.com/acrobat/online/compress-pdf.html

Strictly speaking, no. You will always lose some quality through compression (unless it’s lossless).

Practically, the question you are asking is how much quality are you willing to sacrifice to get to the size you want.

Adobe Acrobat Pro DC has as Save as Other —> Optimised PDF where you can do a space audit and find out why your PDF is too large (it’s usually images) and then you can choose how aggressively you want to reduce their quality to get the file size below a size target.

online pdf compressors are handy for quick jobs, desktop tools better for sensitive files.

for large pdfs, one approach people use is to reduce image resolution or convert images to grayscale before compression, that usually cuts the size significantly without losing much readability. another method i’ve seen discussed is merging duplicate resources in the pdf to avoid redundant data.

Most online tools will probably call ghostscript under the hood:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/prepress -dNOPAUSE -dQUIET -dBATCH -sOutputFile=out.pdf file.pdf

black and white only can make a very small PDF

gs -q -dNOPAUSE -dBATCH -dSAFER -sProcessColorModel=DeviceGray -sColorConversionStrategy=Gray -dDownsampleColorImages=true -dOverrideICC -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dColorImageDownsampleType=/Bicubic -dColorImageResolution=120 -dGrayImageDownsampleType=/Bicubic -dGrayImageResolution=120 -dMonoImageDownsampleType=/Bicubic -dMonoImageResolution=120 -sOutputFile=out.pdf file.pdf

Personally, I like Calibre's ebook-convert. You can get down to really nice sizes by also encoding the images to avif or webp afterwards but it will also mangle the layout so that doesn't sound like what you want !

The problem with most PDF compressors is they just smash image quality without caring about the result. If you're on Mac, there's a desktop app called PDF Compressor that has a 'target size' mode - you set the size you want and it figures out the settings to get there. Keeps images readable while actually hitting the target. Preview's quartz filter does compress but it tends to go nuclear on quality.

If you still need this I actually built a simple tool for it
https://pdfoperations.com

No login and no subscription.

compressor.app handles pretty much any format you throw at it. they also have a dedicated /compress-pdf page.

Most tools just nuke the quality - that's the tradeoff. On Mac, "PDF Compressor - Reduce Sizes" lets you set a target file size so you stay in control. It tries multiple compression levels behind the scenes and picks the best quality it can hit. Way less guesswork than Preview's 'Reduce File Size' which just destroys everything. Ghostscript via Terminal is a solid free option too if you're okay with command line.

Tools like Smallpdf are designed for this use case since their PDF compressor reduces file size by optimizing images and removing redundant data while keeping text and layout intact.

Hi, I have written a post on the topic, as I had the same question and I did some analysis

https://www.reddit.com/r/pdf/comments/1rbum3u/canon_ijscan_utility_pdf_high_compression/

Unfortunatly, there is not a single tool that does everything.

If you need only compression, the best program I found is Canon IJScan Utility with the option "PDF HIGH". 1 Page is compressed up to 70KB without a visibile reduction in quality. The result is impressive because it uses an MSC algorithm which separates the text and the picture and uses the best compression strategy for each object. However it has some drawbacks:

1. You need to own a CANON printer /scanner to install it (I bought a CANON Prixa 7450i).

2. It only works on Windows.

3. Unfortunately the OCR is not so good.

If you instead need a great OCR, the best program is tesseract. It supports multi-language pages and a lot of strange characters and accents. Many programs uses tesseact. The best is ocrmypdf , but also gs can do it. However:

1. They do not uses a MSC compression algorithm. This means the compression is not so good: 1 page is around 800 KB, which is not bad, but still a > 10x

2. They are Linux program, however, as they are open source libraries, I can imagine they are used by Windows programs too.

Ideally it would be great to have program which uses MSC compression algorithm and tesseract for OCR, but I haven't found any.

Many someone has better ideas.